Misc diameter calculator updates (fixes #767, #900, #573)

Respect configured "significant digits" in displayed diameter/circumference values:
* Store actual diameter or circumference value in "data-value" attribute on input element
* Prepare display values using "data-digits" from diameter input element
* All values now computed client-side, simplifying server code (rename reset() to init() and call it from constructor)

Also:
* We now round rather than truncating when displaying all floating-point field values
* Allow entering multiple trunks in "Add Tree" mode
* Show units in diameter and circumference input boxes
* Change "Total trunk diameter" field label to "Trunk diameter"
* Change "Total diameter" message label to "Combined diameter"
* "Combined diameter" message now uses no vertical space when hidden
Merge pull request #927 from RickMohr/diameter-calculator-updates

Misc diameter calculator updates (fixes #767, fixes #900, fixes #573)